Geophysical methods are key tools for exploring the deep structure of fault zones. Different geophysical techniques are sensitive to different physical properties. Commonly, multiple techniques are applied to the same study area with structural results for each technique obtained separately, and the resulting structures compared for interpretation. Joint inversions provide an alternative in which a single structural model is used to try to fit multiple datasets, most commonly with empirical relationships linking the different physical properties (for example, seismic velocity and density). Our work will involve the joint inversion of seismic wave travel time and magnetotelluric profiling data to determine seismic wave velocity and electrical resistivity structure across two segments of the San Andreas Fault in California. One profile passes through the San Andreas Fault Observatory at Depth (SAFOD) where in-situ information is available from the SAFOD borehole, and the other is along the creeping section of the San Andreas near Hollister. This work will improve our understanding of the structure of active transform faults and the complex inherited framework in which they are emplaced. The constrained inversion methods we will develop should have broad applicability to the study of fault zones. The spatial relationship between subsurface structure, seismicity, and hydrologic constraints will result in a better understanding of the primary factors (lithology, fluids, fault geometry) controlling fault behavior.
We will apply a novel variation of the joint inversion approach that involves a constraint where spatial variations in model properties are encouraged to occur at the same places in the model. Specifically, the gradient fields of the physical properties (in our case seismic velocity and electrical resistivity) are computed and the cross product of the two gradients at each cell in the model space is evaluated. Models are "penalized" for having a non-zero cross product, which has the effect of producing models for which a spatial change in one parameter is accompanied by a change in the other that shares the same geometry but can have the same or opposite sign (parallel or anti-parallel gradient), or has no change (zero gradient).
